1. Field of the Invention
This invention relates to a circuit for preventing overdischarge of a rechargeable battery.
2. Description of the Prior Art
In an uninterruptible power system (UPS) which includes an inverter, a rechargeable storage battery electrically coupled thereto and a rectifier coupled between an AC source and the inverter, the power from the AC source is rectified by the rectifier and supplied to the inverter. The inverter converts the rectified power to a dependable AC source for supplying a critical bus. At the same time, the rectified AC power is used to recharge or maintain a constant charge on the battery. In the event that the AC source fails, it would be necessary for the storage battery to provide the back-up power which would be converted to AC for maintaining the AC power supplied to the critical bus. Typically, when lead acid storage batteries are fully charged, they float at, for example, approximately 2.2 volts per cell. Generally, the UPS is required to supply power from its battery for approximately fifteen minutes, and it has been known that the battery cannot be allowed to discharge below a critical end voltage without the possibility of irreversible sulfation beginning to occur. Once the battery discharges beyond the critical voltage, deterioration of the useful life of the battery begins, and if the battery is not recharged within a reasonably short time, it may not be able to be recharged again to its original capacity. In order to protect these batteries, battery manufacturers have provided users with a single value for the final end voltage per cell below which a battery should not be allowed to continue to be discharged. An example of this was shown in the graph in FIG. 2 of U.S. Pat. No. 3,118,137, which shows the end battery voltage to be a constant value regardless of changes or variations in load current. Inasmuch as the battery in UPS systems is an important and expensive component of the UPS system, it has been the desire of UPS manufacturers to ensure that the battery and the UPS would be shut down once the battery discharged to the designated critical value.
Applicants have discovered that if a lead acid storage battery were being discharged at less than rated load, and if this battery were allowed to continue to discharge to a voltage somewhat above a value that had been specified by battery manufacturers as the safe final end voltage per cell of the battery, then, in fact, irreversible sulfation could begin to occur and deterioration of the useful life and capacity of the battery would unexpectedly result. It was applicants further discovery that this final end voltage to which a battery could safely discharge is not a constant in lead acid, and, in particular, in lead-calcium storage batteries, but, in fact, changes inversely with changes in the rate of discharge of current or power from the battery. Both of these discoveries were unexpected, and run contrary to the teaching of the prior art, and, in particular, to that of the above-referred to patent and to U.S. Pat. Nos. 3,895,284, 3,886,442 and 3,778,702. More specifically, the prior art as represented by the above patents generally teach that as the battery discharge current decreases, the capacity of the battery increases, thereby leading one to believe that the battery can effectively deliver power for longer periods of time, with no apparent recognition of the fact that the safe critical final end voltage per cell of the battery actually increases as the discharge current of the battery decreases.